Peter Frost's anthropology blog, with special reference to sexual selection and the evolution of skin, hair, and eye pigmentation

Saturday, July 27, 2013

Skin color and the menstrual cycle

The female torso visibly reddens towards the end of
the menstrual cycle. Do men unconsciously pick up on this visual cue? (Figure
from Edwards and Duntley, 1949)

Women vary in skin color over the menstrual cycle.
From mid-cycle on, their skin steadily reddens because of an increase in blood
flow that peaks in the day or two preceding menstruation.This cyclical
“blushing” mainly affects the torso:

[…] these cyclic variations in
blood flow were observed over the entire trunk and at least the upper parts of
the limbs. There is suggestive evidence that the face and the hands and feet
may share in these changes. (Edwards and Duntley, 1949)

Facial skin, especially around the eyes, shows a
similar cyclical change, according to two questionnaire surveys:

[…] about half the women
questioned had some increase in skin pigmentation, which was noted in every
case in the latter days of the menstrual cycle and in some cases during
menstruation also. The others showed no skin changes whatsoever.

[…] The site most commonly
showing pigmentation changes was the skin around the eyes. Next most frequently
affected were the areola of the nipple and the perioral skin. The forehead,
axilla, and abdomen were affected in less than one-third of the "positive"
subjects. (McGuiness, 1961)

In answer to the questionnaire,
18 women (62%) consistently noticed darkening of the peri-ocular skin towards
the end of the menstrual cycle, i.e. immediately prior to the onset of
menstruation; of these, three also noticed darkening of the nipple areolae, two
the forehead skin and one the peri-oral skin. (Snell and Turner, 1966)

Snell and Turner (1966) confirmed these observations
by measuring the percentage of light reflected by facial skin, although the
cyclical variation was rather small. Unlike the torso, the face doesn’t redden
towards the end of the cycle. Instead, it becomes browner through increased
melanocyte activity:

The results from the skin
reflectance readings did not show any great changes. The readings obtained from
the cheek and lower eyelid indicated that the melanin content of the skin in
these regions tended to rise in the later part of the cycle in many of the
women.

[…] The melanocytes of the
anterior abdominal wall skin over the linea alba showed no changing pattern of
activity at different phases of the menstrual cycle.

[…] It was concluded that a
proportion of normal women, especially dark-skinned brunettes, have darkening
of the facial skin during the later days of the menstrual cycle and this mainly
involves the peri-ocular skin. (Snell and Turner, 1966)

Does this cyclical variation provide men with a means
to assess female fertility? An unconscious means, to be sure. Pierre van den
Berghe thought so, but I ignored his gentle prodding and avoided the subject,
all the more so because a search of the ethnographic literature failed to turn
up any awareness in any human society of this cyclical change. In contrast, many
societies have been keenly aware that women are fairer-skinned and men
darker-skinned, often to the point of making this sex difference an artistic
convention (van den Berghe and Frost, 1986; Frost, 1988; Tegner, 1992).

One research team has tried to find out whether men
pick up on this cyclical variation:

Here, in an initial pilot study,
we test the hypothesis that changes in female facial skin coloration across the
menstrual cycle could be one of the signals that men have adapted to in order
to assess female fertility. Spectrophotometric measurements of the facial skin
color of normally ovulating Caucasian women (aged 24–29 years) were collected
in the late follicular and midluteal phase of their menstrual cycle. Facial
images were also taken in both sessions and judged for attractiveness and
health by a panel of German men (aged 16–37 years). In line with Roberts et al.
(2004), our results show that men perceive women in the late follicular phase
to be significantly more attractive and healthier than those in the midluteal
phase. However, we did not detect any significant differences in objective
measurements of skin color between the two phases. (Samson et al., 2011)

This study suffers from a few flaws. The authors
measured skin reflectance on the forehead and the cheeks, yet these body sites are
less involved in darkening and lightening of female skin over the menstrual
cycle. It would have been better to measure skin reflectance around the eyes
(although premenstrual peri-ocular darkening might have likewise been absent in
the fair-skinned German participants). Better yet, this study should have
focused not on the face but on the torso, since that body region is the one most
affected by this cyclical variation. We should also keep in mind that men
unconsciously use two different aspects of female pigmentation for gender
recognition. One is the lighter skin of a woman’s face. The other is the higher
contrast between facial skin color and eye/lip color (Russell, 2009; Russell, 2010; Porcheron et al., 2013; see also Dupuis-Roy et al., 2009). This contrast
effect might be weakened by the premenstrual darkening of skin around a woman’s
eyes.

A more recent study has corroborated that men prefer
faces of ovulating women to those of premenstrual women (Bobst and Lobmaier, 2012). It concludes that subtle changes in face shape are responsible, although
changes in skin color cannot be ruled out. In fact, if men can respond to such subtle
changes in face shape, they should also be able to respond to changes in facial
color that are no less subtle.

It may be worthwhile to take another look through
the ethnographic literature. One of my wonderful commenters has pointed me to
an article by a sociologist of Zambian origin, Mwizenge S. Tembo:

It is [with] the frequent
circulation of the hormones, the increased flow of blood during ovulation, and
especially during pregnancy that women in Sub-Saharan Africa may acquire a
characteristic mild to quite remarkable red-orangeish glow to their skin. In
fact an obvious tell-tell sign of being pregnant among married women and also
among young girls who may have had sex out of wed lock, even when the pregnancy
is not even physically visible, is the characteristic lightening of the
skin-tone whether the woman is light or very dark. Among the Tumbuka, Chewa, Nsenga, and Ngoni people of Eastern Zambia, several
terms are used to describe the state of being pregnant. “Ali ndi pakati” means that “the woman is in between”. Because the
majority of women in African societies prior to modern medicine had very high
deaths and faced danger during the birthing process, the woman was said to be
literally “between life and death” or “living with uncertainty”. “Ali ndi mimba” means “the woman has a
stomach” referring to the obvious bulging stomach of a pregnant woman. The most
relevant term to this discussion is “ali
ndi pathupi” which means “the woman has a body” (Salaun, 1969; Price, 1970)
which refers to the characteristic light skin tone or the visible obvious glow
the woman assumes when she gets pregnant.

This light reddish skin is considered desirable by
African men and may serve an adaptive purpose: “Among many other possible
explanations, the most compelling may be that the lighter skin, even among the
darkest of indigenous Africans, may have been a normal and natural biological
marker and signal that the woman was very fertile.” Tembo is a fan of
evolutionary psychology and may be indirectly echoing a meme that began with me
and Pierre van den Berghe. Nonetheless, there may indeed be more awareness of
this menstrual change in skin color than I had thought, particularly in
settings where most variation in skin color is intra-ethnic.

Conclusion

When all is said and done, this research topic may still
be ‘a bridge too far.’ Admittedly, a researcher should have little trouble
finding out whether the premenstrual darkening of the eye area is a sexual
turnoff for men. I’m sure it is—many women certainly seem to think so. But how
would one determine whether this male response is hardwired or not? By measuring
it as a function of testosterone levels? Finally, would such a hardwired mental
algorithm shed light on other feelings towards skin color?

It might be more interesting to investigate how men
respond to the premenstrual reddening of the female torso region. Recruitment
of female participants would nonetheless be much more difficult, as would be
the task of getting approval from the research ethics committee. There’s also
the little matter that this premenstrual ‘blushing’ is visible only in
light-skinned women. Conversely, premenstrual darkening of the eye area is
visible mainly in darker-skinned women.

It is in a male's interest to mate with females who are ovulating, but not just with one female. In a monogamous mating system the female has concealed ovulation to encourage the male to stick close all the time, instead of chasing after any ovulating female.

A monogamous man's reproductive fitness would would be maximized by mating with his woman when she was ovulating. When the male has to go on hunting trips he does not want to do that when the woman is ovulating, because there would be a danger of being cuckolded. So men are selected for perceiving subtle signs of ovulation in the skin tone, smell and voice of women as sexually attractive.

Women in the modern world use pseudo-signs of ovulation to maximise their attractiveness. I believe there is even a study that found they were much more likely to wear pink or red when ovulating.

Does the use of contraceptives tie with the sudden preferential flip for tanned skin instead of light skin in women in the west? If contraceptives stop the ruddy effect, sun tanning might be a behavioral adaptation to make up for the lessening of sexual appeal.

"lightly pigmented areolae, indicative of youth and nubility, should receive most visual attention and be rated as most attractive, but images with dark and medium areolar pigmentation are rated as more attractive than images with light areolae"

"There’s also the little matter that this premenstrual ‘blushing’ is visible only in light-skinned women. Conversely, premenstrual darkening of the eye area is visible mainly in darker-skinned women."

And what do you base this on? Your citation of Tembo's paper certainly doesn't indicate that. You even admit yourself you couldn't find any references to this in any human society yourself, but apparently, in african ones, this is widely attested to. This reminds of the claims dark skinned people don't show visible blushing, which I imagine even you're aware is false. With regards to the "darkening of the eye area", the only evidence you've presented is the reference to "dark skinned brunettes", a terribly vague term that would most likely mean southern european women.

Welcome to my blog! For the most part, this page will be an extension of my website, with comments relating to my research. But it will also branch out into more general discussions of human evolution.